Selenium in higher concentrations is very toxic. Leakage of selenium from phosphate, coal and other mining operations and subsequent migration away from the containment area continues to be a significant unresolved problem at a number of government and private storage sites. In phosphate, coal and other mining operations, selenium can leach from the overburden, ore, or mine tailings releasing selenium into surface and ground waters. Selenium is taken up by many plants that can be consumed by wildlife, livestock and people resulting in selenium poisoning. Various techniques are used to try to isolate leaking storage containers and contaminated soil to prevent movement of selenium into uncontaminated soil and especially groundwater. As used herein, the term contaminant and contaminated soil will refer to selenium and selenium contaminated soil, respectively.
One approach is simply to attempt to dig up and remove the selenium contaminated soil. This, however, is costly, and disturbance of contaminated soil carries the risk that some selenium will be missed or released and left to migrate further. Excavation also has a negative effect on soil stability. Excessive digging and excavation around waste tanks, for example, has the potential to aggravate selenium transport by damaging heavily corroded containment drums and disturbing already contaminated soil.
Another approach is to establish an impermeable barrier or seal in the soil of a contaminated site in order to prevent migration of contaminants beyond the barriers. Barriers of this sort that are in use at various sites around the United States and abroad include vertical sleeves of steel or plastic placed in trenches surrounding a site. They also include walls formed through the injection of highly pressurized cementatious grout in holes drilled in the soil. Emplacement of such barriers typically causes considerable disturbance to the soil and often there is no convenient way to create a “floor” or continuous barrier beneath the leaking tank or contaminated region. Consequently, the sequestration of the contaminants is incomplete and contaminants continue to migrate downward and may thereafter migrate outward. For areas under waste tanks, waste trenches and certain geological formations, forming a continuous impermeable barrier or seal is difficult and sometimes impossible.
Another approach is to create a permeable, chemically reactive barrier or zone that selectively actively attracts and chemically binds, sorbs, or traps contaminants (i.e., sequestration), while allowing water and other components or contaminants to pass through unaffected.
These chemically reactive materials can be combined with other components to form slurries that harden in the ground, forming semi-permeable reactive barriers. Jet injection processes, for example, are known and used wherein machines pump slurries in holes drilled around the perimeter of a leaking vessel or contaminated site. Additionally, trenches can be dug and backfilled using chemical sorbent materials. Each of these techniques, however, carries the disadvantages previously mentioned relating to significant disturbance of the soil and difficulty in fully surrounding (or encapsulating) a leaking waste tank or region of contaminated soil.
At this time, no chemically reactive material has been proposed or disclosed for the sequestration of selenium.
A need remains, therefore, for a method for sequestering selenium, and in particular, an in situ method of forming a permeable reactive barrier or zone to selenium.